1. Field of the Invention
The present invention relates to an electromagnetic printing group for a dot matrix printer.
2. Description of the Prior Art
In the dot matrix printers used in the data processing systems, printing is performed by needles or dot printing elements impinging on a printing support. Movement of the needles towards and away from the printing support is caused by printing electromagnets. Movable armature electromagnets of the attraction or release type are generally used.
There are three timing phases in the operation of the actuators. For the attraction electromagnets the phases are as follows:
(A) Energization or impact phase PA0 (B) Return phase PA0 (C) Damping phase.
During the energization phase the electromagnet winding is energized and the movable armature is drawn towards the electromagnetic core. The armature movement causes the movement of a printing needle towards the printing support.
During the return phase the winding is de-energized and the movable armature reverses and returns to its rest position together with the printing needle.
During the damping phase, the movable armature reaches the rest position with a certain speed then interacts with a damping element and assumes a stable rest position with oscillations becoming smaller as the damping action becomes more effective.
The release electromagnet operation also is performed during three timing phases, the last of which is a damping phase. Accordingly effective damping is essential in order to obtain high printing speed performance.
In order to obtain repeatable performance from a printing electromagnet, it is necessary that each time the electromagnet is energized the movable armature is in its rest position. Therefore a printing electromagnet can be energized again only when the damping phase is completed. The maximum actuation frequency of a printing electromagnet is greatly limited by the damping phase duration. Generally the damping of the armature movement is obtained by resilient elements, generally associated to calibration means as disclosed, for example, in U.S. Pat. No. 4,367,962. Among the resilient materials fluoroelastomers are largely used which have high internal viscosity coefficient and therefore develop high damping action. However such damping action is largely affected by the working temperature; at 50.degree. the internal viscosity is greatly reduced. Consequently the dynamic characteristics of the electromagnetic group are negatively affected by the temperature.
Another way to obtain damping action is disclosed in U.S. Pat. No. 4,202,638. This document discloses a pneumatic dampener avoiding the armature bounce in releasing an electromagnet, during the armature return to the rest position. In this case a rigid plate is arranged on the magnetic core poles and the armature, in rest position, lies against such a plate. During the return phase of the armature towards the rest position, the air cushion between the armature and the plate dampens the armature movement and reduces the bounce. This patent indicates that the invention further applies to the attraction type electromagnets. This kind of dampener is not affected by the temperature but is only partially effective.
These undesirable effects are overcome by the present invention where effective and fast damping is obtained by combining the effects of the resilient and pneumatic damping together with a ballistic coupling which enhances the effects and substantially eliminates the armature oscillations.